It is known that the external Electro-Magnetic noise from a motor or a breaker causes a spike noise on the power lines and signals of the LSI will cause functional failures in the LSI.
To prevent this problem, a noise filter has been inserted between the input pad and the input gate of the input buffer as shown in FIG. 1. This noise filter consists of a resistor and capacitor. The resistor connects the input pad and the input gate of the input buffer. The capacitor is inserted between the input gate and the ground of the LSI.
If the noise pulse is applied between the input pad and the ground line, the noise pulse is absorbed by the noise filter and the noise is not detected by the input buffer.
As far as the voltage between VDD and VSS is kept the same (This condition is called the Common Mode) and the time constant of the RC network is big enough, the noise filter of the prior art can prevent the LSI from the mal-function.
However if the following conditions happen, the noise filter of the prior art is not good enough anymore.
(1) The noise filter of FIG. 1 cannot prevent the noise, in case the noise is applied between VDD and VSS (This condition is called the Normal Mode).
(2) High resistance poly-silicon is not available in the deep sub-micron LSI.
(3) The time constant cannot be made big enough when the cycle time is small, in other words the clock frequency is high.